This invention generally relates to electrodes for use in electrochemical processes especially electrowinning processes having a valve metal substrate carrying a semiconductive intermediate coating consisting of tin and antimony oxides with a top coating consisting of lead dioxide to provide a considerably less expensive electrode while obtaining low cell voltages for given current densities. More particularly the present disclosure relates to a much improved electrode having a valve metal substrate such as titanium carrying a semi-conductive intermediate coating consisting of tin and antimony oxides supplied by a series of layers of tin and antimony compounds which are applied, dried and baked to their respective oxides and a top coating consisting of lead dioxide applied by electroplating of the electrode.
Electrochemical methods of manufacture are becoming ever increasingly important to the chemical industry due to their greater ecological acceptability, potential for energy conservation, and the resultant cost reductions possible. Therefore, a great deal of research and development efforts have been applied to electrochemical processes and the hardware for these processes. One major element of the hardware aspect is the electrode itself. The object has been to provide: an electrode which will withstand the corrosive environment within an electrolytic cell; an efficient electrode for electrochemical production; and an electrode cost within the range of commercial feasibility. Only a few materials may effectively constitute an electrode especially to be used as an anode because of the susceptibility of most other substances to the intense corrosive conditions. Among these materials are: graphite, nickel, lead, lead alloy, platinum, and platinized titanium. Electrodes of this type have limited applications because of the various disadvantages such as: a lack of dimensional stability; high cost; chemical activity; contamination of the electrolyte; contamination of a cathode deposit; sensitivity to impurities; or high oxygen overvoltages. Overvoltage refers to the excess electrical potential over theoretical potential at which the desired element is discharged at the electrode surface.
The history of electrodes is replete with examples of attempts and proposals to overcome some of the problems associated with the electrode in an electrolytic cell, none of which seems to have accomplished an optimization of the desirable characteristics for an electrode to be used in an electrolytic cell. Currently, in an electrowinning process for example, the cell is operated at relatively low current densities of less than l ampere per square inch (155 milliamperes per square centimeter). The problem in this case is to find an electrode which will have many of the desirable characteristics listed above and additionally have a low half cell voltage at higher current densities so as to conserve a considerable amount of energy in the electrochemical process. It is know for instance that platinum is an excellent material for use in electrode to be used as an anode in an electrowinning process and satisfies many of the above-mentioned characteristics. However, platinum is expensive and hence has not been found suitable for industrial use to date. Carbon and lead alloy electrodes have been generally used, but the carbon anode has the disadvantage that it greatly pollutes the electrolyte due to the fast wearing and has an increasingly higher electrical resistance which results in the increase of the half cell voltage. The higher half cell voltage causes the electrolytic cell to consume more electrical power than is desirable. The disadvantages of the lead alloy anode are that the lead dissolves in the electrolyte and the resulting solute is deposited on the cathode subsequently resulting in a decrease in the purity of the deposit obtained, and that the oxygen overvoltage becomes too high. Another disadvantage of the lead alloy anode is that the PbO.sub.2 changes to a Pb.sub.3 O.sub.4 which is a poor conductor. Oxygen may penetrate below this layer and flake off the film resulting in particles becoming trapped in the deposited copper on a cathode. This causes a degrading of the copper plating which is very undesirable.
It has been proposed that platinum or other precious metals be applied to a titanium substrate to retain their attractive electrical characteristics and further reduce the manufacturing costs. However, even this limited use of precious metals such as platinum which can cost in the range of about $30.00 per square foot ($323.00 per square meter) of electrode surface areas are expensive and therefore not desirable for industrial uses. It has also been proposed that the surfaces of titanium be plated electrically with platinum to which another electrical deposit either of lead dioxide or manganese dioxide be applied. The anodes with the lead dioxide coating have the disadvantage of comparatively high oxygen overvoltages and both types of coatings have high internal stresses when electrolytically deposited which are liable to be detached from the surface during commercial usage, contaminating the electrolyte and the product being deposited on the cathode surface. Thus, the current density of such anodes is limited and handling of such anodes must be done with extreme care. Another attempted improvement has been to put a layer of manganese dioxide on the surface of a titanium substrate which is relatively porous in nature and building up a number of layers of the manganses dioxide so as to present an integral coating. This yields relatively low voltages as long as the current density remains below 0.5 ampere per square inch (77.5 milliamperes per square centimeter) but as the current density is increased to near 1 ampere per square inch (165 milliamperes per square centimeter) the voltage required rises rather rapidly on this type of electrode, resulting in a considerable disadvantage at higher current densities. Therefore to date none of these proposals have met with much commercial success basically because efficiencies and cost reductions desired have not been achieved to this point.